1. Field of the Invention
The present invention broadly relates to a cross helical rolling mill which is used in the production of seamless pipes such as a seamless steel pipe and, more particularly, to a method of and an apparatus for controlling the operation of such a cross helical rolling mill.
2. Description of the Prior Art
Generally, the piercing step of a seamless steel pipe production line employs a cross helical rolling mill which has a pair of work rolls having an inlet interfacial angle and an outlet interfacial angle, respectively, and arranged at a predetermined lead angle, and guide shoes such as stationary shoes or roller shoes disposed between the work rolls.
FIG. 15 shows the front elevation of a typical conventional cross helical rolling mill. This cross helical rolling mill has a pair of work rolls 1, a plug 2, and a pair of stationary shoes 4. A shell 3, which is shown in section, is tracted by the circumferential frictional forces exerted by the work rolls 1 such as to impinge upon the stationary shoes 4 and slip thereon thereby causing local wear on the surfaces of the stationary shoes 4. In addition, fine cracks which are attributable to thermal stresses are formed on the surfaces of the stationary shoes 4. The local wear and cracks on the shoe surfaces tend to cause seizure between the shell 3 and the stationary shoes 4, which in turn causes scratches called "shoe-scratch" on the surfaces of the rolled shell 3, resulting in a deterioration of the quality of the shell 3. Once this seizure takes place, it is necessary to stop the operation of the rolling mill, in order to repair the stationary shoe 4 or to renew the same. This seriously impairs the production efficiency of the rolling line and increases production costs.
FIG. 16 is a front elevational view of a cross helical rolling mill proposed in Japanese Utility Model Laid-Open No. 60509/1981, in order to obviate the above-described problems caused by the use of the stationary shoes 4. This cross helical rolling mill employs roller shoes 10 which are rotatable so as to guide the shell 3 without any slip between the shell 3 and the surfaces of the roller shoes 10. The roller shoes 10, however, are not power-driven, although they are rotatable. Namely, the roller shoes 10 are idle rollers. Therefore, once the shell is forced into the spaces between the roller shoes 10 and the work rolls 1, the roller shoes 10 do not function to expell the shell 3 from such spaces. In consequence, the rolling has to be stopped because of jamming of the shell 3. This phenomenon is referred to as "sticker". In order to eliminate this sticker, a guide plate 11 may be provided in each of the spaces between the work rolls 1 and the roller shoes 10. The guide plates 11, however, suffer the same problems as those encountered by the stationary shoes mentioned before, so that the advantage derived from the use of the roller shoes 10 cannot be fully enjoyed. If the roller shoes 10 have the same length as the stationary shoes, the roller shoes are deflected by thrust forces due to the excessively lengthy span, so that suitable back-up rolls 12 have to be employed in order to back-up the roller shoes 10.
Thus, in the conventional cross helical rolling mill having guide shoes such as stationary shoes, disk guide shoes and roller shoes, it is not possible to conduct rolling smoothly while stably supporting the material under the rolling.
In order to eliminate these problems, the present inventors have already proposed a cross helical rolling mill, in Japanese Patent Application No. 175211/1983 (Japanese Patent Laid-Open No. 68104/1985). This cross helical rolling mill employs power-driven drive roller shoes arranged obliquely on both sides of the rolling region defined by the work rolls. In this cross helical rolling mill, the drive roller shoes impart a torque to the material under rolling, so that the rolled material is forced towards the adjacent work roll, while being guided by the drive roller shoes. It is, therefore, possible to effect a sxooth rolling with a compact arrangement.
This cross helical rolling mill proposed by the inventors, however, suffers a problem in that the shell 3 on the plug 2 is squeezed into the gap between each work roll and adjacent drive roller shoe 10A as shown in FIG. 17. The squeezing of the shell 3 causes an oscillation of the shell 3, causing an uneven thickness distribution in the circumferential direction, i.e., a lack of uniformity of thickness in the circumferential direction.
On the other hand, the work roll 1 pulls back the squeezed portion of the shell 3, and causes the same to move ahead in a spiral state, so that the rolling efficiency is lowered undesirably.
The tendency for the shell 3 to be squeezed in this way is serious particularly at the rear end portion of the shell 3 because of lack of material portion which would produce a force capable of restraining the deformation. In consequence, the shell 3 tends to be jammed into the gap between the work roll 1 and the drive roller shoe 10A, resulting in an inferior separation of the shell from the rolling mill. The thicker the shell 3 is or the greater the thickness reduction is, the more serious the influence of the squeezing of the shell 3.
The squeezing of the shell 3 is more likely to occur at the leading and trailing ends of the shell 3 than at the central portion of the same. Namely, while the squeezing of the central portion of the shell only results in an uneven circumferential thickness distribution or a reduction in the rolling efficiency, the squeezing at the leading and trailing ends tends to become excessive in amount, often causing jamming of the rolled material between the work roll 1 and the drive roller shoe 10A, and this results in a rolling failure.
It might be possible to increase the torque of the drive roller shoe, in order to limit the amount of squeezing which takes place at the leading and trailing ends of the shell 3. In order to maintain the large torque during the rolling, however, it is necessary to employ a driving motor of large capacity for driving the drive roller shoes.